It doesn't matter. Message listener is one of the modes of communication between your application and end systems, therefore Message listener can be in System layer or can be in experience layer, it depends on who are client of data/information and who are the producer. It means, if client of information wants to exchange data via messaging system, then ideally message listener/sender can be in experience layer, and if producer willing to exchange data via messaging system, then message listener/sender can be in system layer. So, first decide data client/users, and data producer, then put their mode of communication (i.e. API, File, Database connectors, JMS connector etc.) accordingly either in system layer or exp. layer. Any kind of client logic must in exp. layer doesn't matter what mode of connection/communication is..... and any kinds of data producer logic must be in system layer regardless of what modes of communications producers are using. Hope this helps.